1. Field of the Invention
The present invention relates to processes for operating a utility boiler having reduced emissions in the exhaust, minimized/reduced air flow, and reduced slag deposition within. The present invention also relates to methods for reducing emissions from, minimizing air flow into, and reducing slag deposition within a utility boiler.
2. Description of the Related Art
Utility boilers or furnaces are employed in industry for generation of heat, production of steam, and generation of electricity utilizing steam. Utility boilers typically have a furnace therein where a fuel fuel, such as residual oil or #6 fuel oil, is oxidized or burned to generate heat. Along with generating heat, utility boilers will generate or evolve an exhaust gas that will contain carbon dioxide (product of oxidation of fuel oil), residual oxygen (unreacted), inert air components, i.e., nitrogen and argon, and emissions, such as sulfur-based and nitrogen-based compounds. Exhaust gas is typically treated and then vented to the atmosphere.
A problem with operating utility boilers that burn fuel oil is sulfur-based emissions. Sulfur-based emissions result from the oxidation of sulfur impurities in fuel oil. Sulfur-based emissions primarily take the form of sulfur dioxide (SO2), sulfur trioxide (SO3), and sulfuric acid (H2SO4). Sulfur dioxide is the primary oxidation product. Sulfur trioxide occurs primarily from the catalytic oxidation of sulfur dioxide by vanadium compounds present in the fuel oil. Free sulfur trioxide in an exhaust gas imparts an undesirable opaque appearance (a blue haze or trailing plume) to the gas when vented to the atmosphere. Sulfur trioxide can also condense to form sulfuric acid. Free sulfuric acid can cause corrosion of process surfaces in utility boilers as well as acid rain in the atmosphere.
Another problem with operating utility boilers is the presence of vanadium in the fuel oil. Vanadium is commonly employed as a catalyst in the cracking and/or refinement of crude oil and ends up concentrated in the residual oil during the refining process. Vanadium oxidizes during the combustion process to form vanadium pentoxide and related compounds, such as sodium vanadium pentoxide, which are highly corrosive to the process surfaces within the utility boiler. High levels of corrosion reduce the operational life of the utility boiler and associated process equipment. Vanadium pentoxide has the negative effect of catalyzing, and, thus, increasing the conversion of sulfur dioxide to sulfur trioxide, which increases opacity of exhaust gas and increases formation of sulfuric acid.
Another problem with operating utility boilers that burn fuel oil is slag deposition on process surfaces within the utility boiler, including the furnace. Slag deposition can take the form of one or more layers caked/baked onto process surfaces. The one or more layers typically contain metal complexes of vanadium with sodium, nickel, iron, or magnesium. Slag can deposit on the surfaces of tube bundles or other heat transfer devices within the utility boiler denuding the boiler's heat transfer efficiency. Ultimately, slag deposits must be removed to restore the operational and heat exchange efficiency of the utility boiler.
One means employed in the prior art to address the aforementioned problems is to employ a slag control agent in the combustion process. Conventional slag control agents are typically magnesium compounds, such as magnesium hydroxide and magnesium oxide. The slag control agent reacts with and neutralizes sulfuric acid to form innocuous, non-corrosive compounds thereby reducing acid emissions and corrosion of process surfaces. The slag control agent also reacts or complexes with vanadium compounds, such as vanadium pentoxide and sodium orthovanadate, to render them more friable, non-corrosive and removable via conventional cleaning methods. A problem with the use of a slag control agent is that relatively large amounts may be required if the acid levels in the utility boiler are high. Such large amounts of slag control agent may cause an unacceptably high degree of slag deposition, reflectivity (reflection of the radiation component of heat) or buildup on the process surfaces of the utility boiler.
Another problem with operating utility boilers that burn fuel oil is nitrogen-based emissions. Nitrogen-based emissions (NOx) result from the oxidation of trace amounts of nitrogen present in the air within the utility boiler. NOx emissions include nitrous oxide (NO) and nitrogen dioxide (NO2).
Another problem with operating utility boilers that burn fuel oil is particulate emissions. Particulate emissions are made up of unburned carbon and ash. Unburned carbon is formed when burning of oil in the boiler is incomplete. Ash is inorganic particulates present in fuel oil leftover from the oil refining. The amount of unburned carbon is controllable and can be impacted by regulation of operation of the boiler. The amount of ash is not controllable since it cannot be burned.
A means employed in the prior art to address the problems of excess particulate emissions is to increase the amount of excess air in the boiler. The excess air reduces the amount of unburned carbon. A drawback to increasing the amount of excess air in the boiler is that NOx emissions are exacerbated or increased. Thus, controlling NOx emissions and particulate emissions are competing aims. Another drawback to increasing the amount of excess air is that the amount of heat lost to exhaust is increased. Thus, operational efficiency of the boiler is compromised.
It would be desirable to have a process for operating a utility boiler, wherein the boiler exhibited reduced sulfur-based, nitrogen-based, and particulate emissions. It would also be desirable to have a process for operating a utility boiler that exhibits reduced slag deposition and affords enhanced operational efficiency.